analysis: add GN toolchain analysis

toolchain-analysis/gn.md

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+# GN toolchain analysis
+
+[GN](https://gn.googlesource.com/gn/) is a meta-build system developed by Google
+that generates build files for Ninja. It is currently the official build system
+for V8 and Chromium, it is used to build Node.js on Fuchsia, and is also used by
+open source projects like Electron.
+
+## Supported platforms
+
+TODO: test GN against currently Node.js supported platforms and toolchains (see
+[BUILDING.MD](https://github.com/nodejs/node/edit/master/BUILDING.md)).
+
+### Unsupported platforms
+
+TODO: see above
+
+## Pros
+
+* Easier to upgrade V8 as we don't need to rewrite build configs for V8
+* V8, Fuchsia and Electron already have GN files for Node.js. Unifying those in
+  the Node.js repository might be easier than writing build files for a
+  different toolchain
+* Projects integrating Node.js and Chromium don't need to maintain their own GN
+  build files for Node.js
+* Extensibility should allow implementation of custom features or platforms
+
+## Cons
+
+* Other dependencies will need Node.js-maintained GN build files
+* Chromium team does not recommend using GN outside Chromium [^1]
+* V8 team can't guarantee GN will still be maintained
+* Add extra dependencies (`gn`, `ninja`, `build`, `buildtools`, etc.)
+* `depot_tools` is cumbersome for non-Google engineers, but not using it means
+  we would be using GN in a non-supported way
+
+[^1]: Electron team had good experiences asking GN team for help in the past
+
+## Known ports
+
+* [node-ci](https://chromium.googlesource.com/v8/node-ci/): Node.js built with
+  GN to run on Google CI infrastructure
+* [electron patch](https://github.com/electron/electron/blob/main/patches/node/build_add_gn_build_files.patch)
+  to add GN build files to Node.js
+* Fuchsia Node.js build uses GN
+
+
+## Need more info
+
+* Can GN be used in a hybrid approach where other dependencies are built with
+  their own toolchains (like CMake)?
+* How well does it work with gcc and other non-clang compilers?
+* How to generate the files needed for building native modules, assuming GN will
+  not be used for native modules?
+* Using GN means we'll also be using ninja instead of make?
+
+## Observations
+
+* If agreed among projects, burden of GN build files maintenance can be shared
+  between Node.js, V8, Electron, and any other interested projects.
+* There is also the possibility of using GN to build V8 alone and use something
+  else for the rest of the project
+* Assuming we land on another toolchain, would it stil be possible to have GN
+  build files on the project to better integrate with V8 tests as well as
+  Electron and Fuchsia build workflows?